---
title: "Seed demo data and make preview databases"
description: "Seed the same synthetic demo rows every time, and give each git branch a preview database that expires after its time to live."
section: "guides"
status: "Planned for 0.4"
spec: "seed-preview"
canonical: "https://mainsdk.dev/docs/guides/seed-and-preview"
updated: "2026-10-28"
---

# Seed demo data and make preview databases

> Seed the same synthetic demo rows every time, and give each git branch a preview database that expires after its time to live.

Status: Planned for 0.4 · Canonical: https://mainsdk.dev/docs/guides/seed-and-preview · Updated: 2026-10-28

You want realistic demo data on your laptop and in preview environments, with no real personal data in it. This guide seeds a database from a number, gives each git branch its own preview database, and removes old previews. It takes about 20 minutes.

Planned for 0.4 (spec `seed-preview`).

## You need

- A MainSDK app with `db` models.
- A database that can be seeded: a local one, or a preview database. Production is refused.

## 1. Describe the demo data

```yaml title="main.yaml"
seed:
  number: 42                    # the same number gives the same rows
  tenants: 3
  regions: [eu, in, us]
  per-tenant: { users: 20 }
  models:
    orders:
      count: 50
      fields:
        status: [paid, shipped, refunded]
        note: { text: short }
preview:
  driver: local                 # neon | supabase | local
  from: main
  seed: true
  ttl: 7d
```

Use reserved values for anything that looks like a person. An email must use a reserved domain, such as `example.com`, or `MAIN-SEED-0011` fails the check.

## 2. Seed a local database

```bash
npx mainsdk seed --number 42 --tenants 3
```

The command prints a `digest`. The same number, schema and SDK major always give the same digest. Use `--reset` to clear the seeded rows first.

## 3. Create a preview for the branch

```bash
npx mainsdk preview up
```

The preview is named after the current git branch and is seeded with the same number. Running `preview up` again on the same branch returns the same preview.

## 4. Clean up

```bash
npx mainsdk preview list
npx mainsdk preview prune
```

`prune` deletes previews whose branch is gone or whose `ttl` has passed. It never deletes the parent database.

## Check it worked

- `mainsdk seed` prints a digest, and running it twice with `--reset` prints the same digest.
- `mainsdk preview up` prints the preview's name and its database secret reference.

## If it fails

- `MAIN-SEED-0001`: the environment is production. Seed a local or preview environment.
- `MAIN-SEED-0002`: the target database is not seedable. Point at a local or preview database.
- `MAIN-PREVIEW-0001`: the db driver cannot branch. Use `preview.driver: local`.
- `MAIN-PREVIEW-0002`: the provider failed while making the preview. Check its status and credentials, then run `preview up` again.

## Prompt for your coding agent

> Seed this app with the same demo data every time, using MainSDK's `seed` and `preview` interface. Use reserved example emails only, give each git branch a preview database that expires after seven days, and make `preview prune` part of the nightly job.